The present invention generally relates to an electronic circuit that controls a signal coupled to a load. It more particularly relates to a digital pulse width modulator using a delta sigma modulator as an input stage to control a drive signal to a load.
Industry has long used pulse width modulators (PWM) to supply and control drive to motors, direct-current to alternating-current conversion, power supply regulators, and many other power loads. As is well known, the duty cycle of a square wave may be varied or integrated to create a desired magnitude of an output alternating-current signal. Due to the non-zero switching time in power switching devices, the number of digital transitions per unit time typically determines the thermal power dissipation of systems using these devices. PWM circuits have a fixed number of transitions per unit time.
Traditional PWM applications use a triangle or sawtooth reference waveform to compare against an incoming analog signal (which may, among other things, be an adjustment signal or a feedback signal or both). The digital result of this comparison is a PWM output. Generally, the generation of such an analog triangle or sawtooth reference waveform has been accomplished using precision capacitors, resistors, and current sources as a charge/discharge ramp up/down circuit. While this implementation is not a problem for discrete realizations, implementing such components in an integrated circuit results in components that are hard to control. The components may exhibit non-linear behavior, especially if the integrated circuit technology is optimized for digital operation.
PWM technology is relatively simple, producing a PWM output signal that closely approximates the input signal (within the frequency band of interest), and has a fixed number of digital transitions per unit time. Unfortunately, the fixed frequency nature of traditional PWM technology results in a significant amount of spurious harmonic noise signals which can result in significant electromagnetic interference to other electronic circuits.
Overcoming the problem of harmonic noise has been addressed before, with most methods spreading the harmonic noise by dithering the PWM fundamental frequency or introducing extra transitions in the PWM waveform to tailor the spectrum. This spectrum tailoring, however, does not retain the PWM feature of a fixed number of digital transitions per unit time, which is important for some applications.
Delta sigma modulation (DSM) has been used extensively in analog-to-digital and digital-to-analog conversion, and inputs are typically sampled at a frequency several times higher than the input signal frequency. (Delta sigma modulation is occasionally referred to as sigma delta modulation. The terms are considered interchangeable herein). DSM has the advantage of allowing the shaping of quantization noise in such a way as to significantly reduce the quantization noise at low frequencies relative to the frequency of the switching clock. The output from a DSM circuit is a stream of digital pulses. An average of these digital output pulses over a finite period can be used to represent the input signal, if the averaging period chosen is at least half of the period of the input signal. An additional advantage of DSM technology is that DSM circuits can be built using standard digital CMOS processes for integrated circuits. This is because DSM circuits are usually implemented as switch capacitor circuits which rely on precise capacitor matching rather than absolute capacitor values. A disadvantage of DSM circuits is that the clock frequency employed can be 100 times the frequency of the input signal. This high oversampling of the input signal can be reduced by using alternative approaches by selecting the proper DSM and averaging technique. The high frequency DSM clock results in many more transitions per second, than traditional PWM. Operating a DSM at lower clock frequencies results in more quantization noise in the frequency band of interest, which interferes with resolution of the signal. This increased quantization noise and increased number of transitions per second has not encouraged the use of DSM when a PWM technology has been available.
Thus, in order to overcome the problems associated with the harmonic noise generation and the need for triangle or sawtooth analog signal generation, a unique combination of PWM and DSM technologies has been created.